1. Field
The present invention relates generally to telecommunication systems, and more specifically to scheduling techniques in such systems.
2. Background
Third-generation (3G) data services for both code-division multiple access 2000 (cdma2000) and wideband code-division multiple access (WCDMA) were designed around a physical layer framework that was optimized for voice transport. Voice services are characterized by symmetric traffic loading (between the forward link and the reverse link) and guaranteed quality of service (latency, delay, etc.). Packet data services, however, are characterized by asymmetric traffic loading that involves short messages (e.g., requests for data) and very long messages (e.g., data downloads). Therefore, using a voice-traffic optimized system for packet data services results in degradation of spectral efficiency or reduction in the economics of such services.
Consequently, Third-Generation Partnership Project 2 (3GPP2) has adopted an evolutionary approach to the existing cdma2000 standard for wireless Internet services. The high data rate (HDR) system, also known as 1×evolution (1×EV) Phase 1, is an evolution of the cdma2000 family of standards and is designed to provide an air interface for packet data applications such as wireless Internet with peak rates of up to 2.4 Mbps.
The 1×EV forward link includes the pilot channel, the medium access control (MAC) channel, the forward traffic channel and the control channel. The traffic channel carries user data packets. The control channel carries control messages and may carry user traffic. These channels are time division multiplexed in order to support accurate pilot-based signal-to-interference-and-noise ratio (SINR) measurements at the mobile, to support full power transmission of the traffic channel to a single mobile, and to support code division multiplexing of the low data-rate MAC channels.
Adjacent cells in an HDR system are typically allocated the same frequency. However, the adjacent cells may not interfere with each other because cells use different codes that are orthogonal to each other.
Unfortunately, interference does occur in an HDR system for a number of reasons. Antenna patterns, power levels, scattering, and wave diffraction can differ from cell to cell. Buildings, various other structures, hills, mountains, foliage, and other physical objects can cause signal strength to vary over the region covered by a cell and to create multipath. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile can vary widely within a cell and from cell to cell. For this reason, cells adjacent one another do not typically form the precise geometric boundaries. Since cell boundaries must overlap to provide complete coverage of an area and allow handoff, and because the boundaries of cells are imprecisely defined, signals will often interfere with one another. This is especially true when a sectored cell pattern is used, because the transmitters in each of the cells are much closer to one another than in a simple cell pattern. Further, although the system may be less than fully loaded, the existing scheduler may needlessly schedule transmission in adjacent cells or sectors simultaneously because transceivers of the adjacent cells are typically configured to transmit starting at the same point in time.
FIG. 1 shows example timelines of conventional schedulers that schedule transmission times for four transceivers (i.e., transceivers 1–4) of adjacent cells. As can be seen, the conventional schedulers typically schedule transmission times for the four transceivers in the adjacent cells within a particular time period (e.g. T) starting at the same point in time (e.g.,τ0) regardless of the amount of data to be transmitted for each transceiver. The overlapping of the transmission times, which are scheduled by conventional schedulers, occurs because the transceivers of the adjacent cells transmit data substantially simultaneously. For example, in FIG. 1, the base station transceivers in cells 1 and 3 have only about a quarter of T worth of data to be transmitted (i.e., approximately 25% loaded) during the first T period while the base station transceivers in cells 2 and 4 have about a third of T worth of data to be transmitted (i.e., approximately 33% loaded). Nevertheless, the transmissions are scheduled to begin at the same time. Thus, in this configuration, interference between the cells at the boundaries may be relatively high because, for a significant portion of the transmission period (T), all four transceivers are simultaneously transmitting.
There is therefore a need in the art for a modified scheduling technique in transmission of data packets among adjacent cells in such a way as to reduce interference, especially when the system is not fully loaded.